with structuredAttrs lists will be bash arrays which cannot be exported
which will be a issue with some patches and some wrappers like cc-wrapper
this makes it clearer that NIX_CFLAGS_COMPILE must be a string as lists
in env cause a eval failure
Without the change -fno-common toolchain like llvm-11 fails as:
ld: libcamlrun.a(minor_gc.o):/build/ocaml/byterun/caml/major_gc.h:67: multiple definition of
`caml_major_ring'; libcamlrun.a(stacks.o):/build/ocaml/byterun/caml/major_gc.h:67: first defined here
